1. Field of the Invention
This invention relates to a method and an apparatus which effect recording on the surface of a recording medium by a recording head which scans the surface of the recording medium in a main scanning direction and a sub-scanning direction.
2. Description of the Related Art
Recording apparatuses which effect recording while causing a recording head to scan in a main scanning direction and a sub-scanning include, for example, the serial scan printer and the drum scan printer. As taught by U.S. Pat. No. 4,198,642 and Japanese Patent Laying-Open Gazette No. 53-2040, for example, a technique called the "interlace scheme" has been developed for improving the image (print) quality of printers of this type, particularly those that employ ink jet nozzles.
FIG. 22 is a diagram for explaining an example of the interlace scheme. In this specification, the following parameters are used to define printing scheme.
N: Number of nozzles PA1 k: Nozzle pitch PA1 s: Number of scan repeats PA1 D: Nozzle density nozzle/inch! PA1 L: Sub-scanning pitch inch! PA1 w: Dot pitch inch!
The number of nozzles N is the number of nozzles used to form dots. In the example of FIG. 22, N=3. The nozzle pitch k is the interval between the centers of the recording head nozzles expressed in units of the recorded image pitch (dot pitch w). In the example of FIG. 22, k=2. The number of scan repeats s is the number of main scans in which all dot positions on a main scanning line are serviced. In the example of FIG. 22, s=1, i.e., all dot positions on a main scanning line are serviced in a single main scan. When s is 2 or greater, the dots are formed intermittently in the main scanning direction. This will be explained in detail later. The nozzle density D (nozzle/inch) is the number of nozzles per inch in the nozzle array of the recording head. The sub-scanning pitch L (inch) is the distance moved in 1 sub-scan. The dot pitch w (inch) is the pitch of the dots in the recorded image. In general, it holds that w=1/(D.multidot.k), k=1/(D.multidot.w).
The circles containing two-digit numerals in FIG. 22 indicate dot recording positions. As indicated in the legend, the numeral on the left in each circle indicates the nozzle number and the numeral on the right indicates the recording order (the number of the main scan in which it was recorded).
The interlace scheme shown in FIG. 22 is characterized by the configuration of the nozzle array of the recording head and the sub-scanning method. Specifically, in the interlace scheme, the nozzle pitch k indicating the interval between the centers of adjacent nozzles is defined as an integer not smaller than 2, while the number of nozzles N and the nozzle pitch k are selected as integers which are relatively prime. Further, sub-scanning pitch L is set to N/(D.multidot.k) (=N.multidot.w).
The interlace scheme has the advantage of enabling nonuniformity of nozzle pitch, ink jetting characteristic and the like to be spread out over the recorded image. Because of this, it improves image quality by mitigating the effect of any nonuniformity that may be present in the nozzle pitch, the jetting characteristic and the like.
The shingling scheme, also known as the multiscan scheme, taught for example by Japanese Patent Laying-Open Gazette No. 3-207665 and Japanese Patent Publication Gazette No. 4-19030 is another technique used to improve image quality in color ink jet printers.
FIG. 23 is a diagram for explaining an example of the shingling scheme. In the shingling scheme, 8 nozzles are divided into 2 nozzle sets. The first nozzle set is made up of 4 nozzles having even nozzle numbers (left numeral in each circle) and the second nozzle set is made up of 4 nozzles having odd nozzle numbers. In each main scan, the nozzle sets are each intermittently driven to form dots in the main scanning direction once every (s) dots. Since s=2 in the example of FIG. 23, a dot is formed at every second dot position. The timing of the driving of the nozzle sets is controlled so that the each nozzle set forms dots at different positions from the other in the main scanning direction. In other words, as shown in FIG. 23, the recording positions of the nozzles of the first nozzle set (nozzles number 8, 6, 4, 2) and those of the nozzles of the second nozzle set (nozzles number 7, 5, 3, 1) are offset from each other by 1 dot in the main scanning direction. This kind of scanning is conducted multiple times with the nozzle driving times being offset between the nozzle sets during each main scan to form all dots on the main scanning lines.
In the shingling scheme, the dots of each main scanning line are not all recorded by the same nozzle but by multiple nozzles. Even when the nozzle characteristics (pitch, jetting characteristic etc.) are not completely uniform, therefore, enhanced image quality can be obtained because the characteristics of the individual nozzles is prevented from affecting the entire main scanning line.
Securing high image quality with a printer or other recording apparatus requires prevention of image quality degradation by variance (nonuniformity) of the nozzle pitch, the jetting characteristic and the like. Conventionally this has been achieved by use of the aforesaid interlace scheme or shingling scheme.
A simple combination of the interlace scheme and the shingling scheme is problematic because it makes the scanning speed difficult to increase. This is because use of the shingling scheme requires the recording head to be supplied with a drive signal Sdot' (FIG. 23) capable, for example, of distinguishing both the dot positions of the even-numbered nozzles and the dot positions of the odd-numbered nozzles. This means that the recording head has to be supplied with a drive signal Sdot' capable of distinguishing all dot positions in the main scanning direction.
When the main scanning speed of the recording head (the carriage speed) is increased, the drive frequency of the drive signal Sdot' has to be increased in proportion. Moreover, the print data sent to the printer from a personal computer or other print data generating device have to be sent to the printer as data corresponding to the individual dot positions. Up to now, therefore, the recording head main scanning speed has sometimes been limited by the nozzle drive frequency and/or the print data supply speed.